Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. One type of MRAM is a spin transfer torque random access memory (STT-RAM). STT-RAM utilizes magnetic junctions written at least in part by a current driven through the magnetic junction. A spin polarized current driven through the magnetic junction exerts a spin torque on the magnetic moments in the magnetic junction. As a result, layer(s) having magnetic moments that are responsive to the spin torque may be switched to a desired state.
For example, FIG. 1 depicts a conventional magnetic tunneling junction (MTJ) 10 as it may be used in a conventional STT-RAM. The conventional MTJ 10 typically resides on a bottom contact 11 and uses conventional seed layer(s) 12. The conventional MTJ 10 includes a conventional free layer 14, a conventional tunneling barrier layer 16, a conventional polarization enhancement layer (PEL) 18, a conventional reference layer 22 and a conventional capping layer 26. Also shown is top contact 28. The conventional PEL 18, conventional Ta spacer layer 20 and conventional reference layer 22 may be considered to form a conventional reference stack.
Conventional contacts 11 and 28 are used in driving the current in a current-perpendicular-to-plane (CPP) direction, or along the z-axis as shown in FIG. 1. The conventional seed layer(s) 12 are typically utilized to aid in the growth of subsequent layers having a desired crystal structure. The conventional tunneling barrier layer 18 is nonmagnetic and is, for example, a thin insulator such as MgO.
The magnetic moments of the conventional free layer 14 and the conventional reference layer 22 are substantially perpendicular to plane (i.e. in the z-direction). The reference layer 22 is a synthetic antiferromagnet (SAF) that includes two magnetic layers 23 and 25 separated by a nonmagnetic layer 24 that mediates an RKKY interaction. The nonmagnetic layer 24 is typically Ru. The layers 23 and 25 are antiferromagnetically coupled through the Ru layer 24, which reduces the external field at the free layer 14. The perpendicular magnetic anisotropy, Hk, of the layers 23 and 25 and the free layer 14 exceeds the out-of-plane demagnetization energy of the layers 23, 25, and 14, respectively. Thus, their magnetic moments are perpendicular as shown in FIG. 1. Typically, the magnetic layers 23 and 25 are actually multilayers including both Co layers and Pt or Pd layers. For example, the magnetic layers 23 and 25 may include a CoPd multilayer (layers of Co interleaved with layers of Pd), a CoPt multilayer (Co layers interleaved with Pt layers), or both. In addition, other constituents such as additional co and/or Fe layers may be included. These magnetic multilayers have a perpendicular anisotropy that is sufficient for the reference layer 22 to remain stable during use of the magnetic junction 10. In contrast, the magnetic moment of the conventional free layer 14 is changeable. This is represented by the dual headed arrow 15 in FIG. 1.
The conventional PEL layer 18 enhances the spin polarization of a current passing in the perpendicular (e.g. z) direction. The conventional PEL typically consists of magnetic materials. For example, a CoFeB layer, an Fe layer, or a CoFeB layer adjoining an Fe layer are typically used. The conventional PEL layer 18 is magnetically coupled with the reference layer 22 in order to ensure the magnetic stability of the conventional PEL 18.
For the conventional magnetic junction 10, a high signal is desired. Thus, the tunneling magnetoresistance (TMR) is desired to be large. A large TMR is generally associated with a high quality conventional tunneling barrier 16. The conventional tunneling barrier 16 is typically crystalline MgO with a (100) orientation. In addition, a relatively small lattice mismatch between the MgO and adjoining ferromagnetic layers 14 and 18 is desired to maintain the perpendicular anisotropy of the layers 14 and 18. For example, CoFeB or Fe are typically used for the layers 14 and 18.
The conventional Ta spacer layer 20 is used to ensure that the conventional reference layer 23 and conventional PEL 18 have independent crystalline orientations. The conventional Ta spacer layer 20 also reduces the pinning field between the layers 18 and 23. In addition, the conventional Ta spacer layer may prevent diffusion of materials, such as Ru and Pd, from the reference layer 22 to other layers of the magnetic junction 10. More specifically, the Ta spacer layer 20 prevents diffusion of Ru and Pd from the layer 24 to the tunneling barrier layer 16. Diffusion of Ru and/or Pd into the tunneling barrier layer adversely affects the TMR of the conventional magnetic junction 10. It is believed that the diffusion of Ru degrade the MgO layer 16 and cause the MgO layer 16 to have a crystalline orientation other than the desired (100) texture. The conventional Ta spacer layer 20 thus has a thickness that is at least sufficient to prevent diffusion of materials such as Ru and Pd from the reference layer 22 to the PEL layer 18 and the MgO tunneling barrier layer 16. It is believed that the conventional Ta spacer layer 20 is at least four Angstroms thick in order to function as a diffusion barrier. In the conventional magnetic junction 10 shown, the conventional Ta spacer layer 20 also allows for magnetic coupling, such as RKKY coupling, between the PEL 18 and the magnetic layer 23.
To switch the magnetization 15 of the conventional free layer 14, a current is driven perpendicular to plane (in the z-direction). When a sufficient current is driven from the top contact 28 to the bottom contact 11, the magnetization 15 of the conventional free layer 14 may switch to be parallel to the magnetization 18 of the conventional PEL 18. When a sufficient current is driven from the bottom contact 11 to the top contact 28, the magnetization 15 of the free layer 14 may switch to be antiparallel to that of the PEL layer 18. The differences in magnetic configurations correspond to different magnetoresistances and thus different logical states (e.g. a logical “0” and a logical “1”) of the conventional MTJ 10. Thus, by reading the tunneling magnetoresistance (TMR) of the conventional MTJ 10 the state of the conventional MTJ can be determined,
Although the conventional MTJ 10 may be written using spin transfer, read by sensing the TMR of the junction, and used in an STT-RAM, there are drawbacks. In particular, the stability or TMR of the conventional MTJ 10 may be poorer than is desired. The conventional PEL 18 may be magnetically coupled to the conventional magnetic layer 23 through the conventional Ta spacer layer 20. However, it is well known that this coupling through Ta may be relatively weak. For example, the RKKY coupling of Ta is expected to be orders of magnitude less than that for Ru. In addition, it is believed that the coupling through the Ta spacer layer 20 is due to a mechanism such as pinhole or orange peel. Such a coupling mechanism is unpredictable and may vary across a wafer. This may result in variation between individual memory cells in a magnetic memory. Although thermally stable, the conventional PEL 18 may have its magnetic moment switch direction during operation of the magnetic junction. Stated differently, the magnetic moment of the conventional PEL 18 may not be as stable as desired for some of the conventional magnetic junctions 10 in a particular magnetic memory. As a result, performance of the conventional MTJ may be adversely affected.
Accordingly, what is needed is a method and system that may improve the performance of the spin transfer torque based memories. The method and system described herein address such a need.